Monoclonal antibodies have great potential for numerous therapeutic purposes. The advantages of monoclonal antibody therapeutics over conventional pharmaceuticals include their exquisite selectivity, multiple effector functions, and ease of molecular manipulation such as radio-isotope labelling and other types of conjugation. A wide variety of target antigens have been used to generate specific monoclonal antibodies. See for example Therapeutic Monoclonal Antibodies, C. A. K. Borrebaeck and J. W. Larrick eds., Stockton Press, New York, 1990, and The Pharmacology of Monoclonal Antibodies, M. Rosenberg and G. P. Moore eds., Springer-Verlag, Berlin, 1994.
One therapeutic application of monoclonal antibodies is passive immunotherapy in which the exogenously produced immunoglobulins are administered directly to the animal being treated by injection or by ingestion. To be successful, passive immunotherapy must deliver an appropriate amount of an immunoglobulin to the animal, because passive immunotherapy does not rely on an immune response in the animal being treated. The immunoglobulins administered must be specific for the pathogen or molecule desired to effect treatment. One advantage of passive immunotherapy is the speed at which the antibody can be contacted with the target compared to a normal immune response. Passive immunotherapy can also be used as a prophylaxis to prevent the onset of diseases or infections.
A major potential use of passive immunotherapy is in combating bacterial infections. Recent emergence of antibiotic resistant bacteria make treatment of bacterial infections with passive immunotherapy desirable. Antibiotic treatment targeted to a single pathogen often involves eradication of a large population of normal microbes, and this can have undesired side effects. An alternative approach has been to utilize the inherent specificity of immunoglobulins to inhibit a specific pathogenic function in very specific microbial populations. In this strategy, purified immunoglobulins of the appropriate specificity would be administered in order to provide a passive barrier to pathogen invasion.
In addition, the immunoglobulins used for passive immunotherapies for example, for oral administration of immunoglobulins must meet certain requirements. First, the immunoglobulin must be functional in very harsh environments, such as the gastrointestinal tract. Second, the immunoglobulin must be resistant to the actions of proteases so that it will not be degraded prior to inactivating the target.
Certain types of cells, including epithelial cells and hepatocytes, are capable of assembling immunoglobulin molecules which have been specifically adapted to function in harsh environments. These immunoglobulins are referred to as secretory immunoglobulins (SIg) and include both secretory IgA (SIgA) and secretory IgM (SIgM). The protection provided by endogenous secretory immunoglobulins have been demonstrated. Several mechanisms for protection from bacterial infection by secretory immunoglobulins have been proposed, including, but not limited to, direct killing, agglutination, inhibition of epithelial attachment and invasion, inactivation of enzymes and toxins, opsonization, and complement activation. In an animal, endogenously produced SIgA are exposed to very harsh environments where numerous proteases, such as intestinal and bacterial enzymes are extremely active and denaturants, such as stomach acid, are also present.
One component of secretory immunoglobulins, the secretory component, helps to protect the immunoglobulin against these inactivating agents thereby increasing the biological effectiveness of secretory immunoglobulin.
The mechanism of synthesis and assembly of these secretory immunoglobulins, such as SIgA or SIgM is extremely complex. In animal cells, secretory immunoglobulins are assembled in a process involving different cell types. Each secretory immunoglobulin is made up of immunoglobulin heavy and light chains, joining chain (J chain) and a secretory component. The immunoglobulin producing B cells make and assemble the immunoglobulin heavy and light chain together with J chain to produce dimeric or polymeric IgM or IgA. The secretory component is produced by a second type of cell, either epithelial cells or hepatocytes, and secretory immunoglobulin is assembled in and secreted from these cells. The mechanism by which these cells assemble and secrete the secretory immunoglobulin is extremely complex and requires a unique microenvironment provided, for example, by mucosal tissues. The microenvironment places the B cells that produce the polymeric immunoglobulin near the cells that assemble and secrete secretory immunoglobulin onto the mucosal surface of an animal.
The epithelial cells have a receptor, the polyimmunoglobulin receptor (pIgR), that specifically recognizes and binds polymeric immunoglobulin/containing J chain, internalizing it and transporting it through the epithelial cell. Expressed on the basolateral cell surface, the pIgR has an N-terminal signal peptide of 18 amino acids, an extracellular polyimmunoglobulin binding portion of 629 amino acids, a membrane spanning segment of 23 hydrophobic residues, and a cytoplasmic tail of 103 amino acids. The extracellular portion contains five immunoglobulin-like domains of 100-111 amino acids each and constitutes the secreted form of the molecule. See for example, Mostov, Ann. Rev. Immol., 12: 63-84 (1994). The site at which the polyimmunoglobulin receptor is cleaved to generate mature secretory component has not been accurately determined.
The polyimmunoglobulin receptor is located on the basolateral surface of epithelial cells in animals. Polymeric, J chain-containing immunoglobulins produced in B cells interact with and are bound by the receptor resulting in vesicularization, transport across the epithelial cell, and ultimate secretion to the mucosal surface. Transepithelial transport also involves proteolysis and phosphorylation to produce the mature SIg containing the secretory component. The close association of the required cells found in the mucosal microenvironment, specifically the B lymphocytes and epithelial cells, is required for secretory immunoglobulin assembly.
The targeting of the production of immunoglobulins in transgenic organisms, such as mice, is extremely difficult and transgenic organisms made from fungus or plants do not contain the proper cell types and mucosal microenvironment to produce secretory immunoglobulins. The production of large amounts of secretory immunoglobulins in transgenic organisms and cell culture has, before this invention, been impossible. One desiring to produce a secretory immunoglobulin in cell culture or a transgenic organism must express the immunoglobulin heavy chain, the immunoglobulin light chain, and J chain in a B lymphocyte. To mimic the proper mucosal microenvironment a cell having the pIgR receptor on its surface would also have to be present and be in close association with that B lymphocyte to even attempt to assemble a functional secretory immunoglobulin.
This elaborate process required for natural secretory immunoglobulin assembly is extremely difficult to duplicate in cell culture or transgenic organisms. Production of SIg in cell culture or transgenic organisms would require coupling the functions of cells producing immunoglobulin with the functions of epithelial cells in artificial (in vitro) systems. Moreover, if the desired transgenic organism is a fungus, a bacterium, or a plant, the cell types and pathways of receptor-mediated cellular internalization, transcytosis, and secretion simply are not present. Those organisms lack epithelial cells and the required mucosal microenvironment.
To date only the assembly of immunoglobulins having light, heavy and J chain within the same cell has been reported. See Carayannopoulos et al. Proc. Nat Acad. Sci,. U.S.A., 91:8348-8352 (1994). However, the assembly of an immunoglobulin having the additional protein component, secretory component, within a single cell has not been described.
The present invention discloses a novel method for the assembly of these complex molecules. Rather than assemble the tetrameric complex at the epithelial cell surface by the interaction of a membrane bound polyimmunoglobulin receptor with immunoglobulin, we have assembled secretory immunoglobulin composed of alpha, J, and kappa immunoglobulin chains associated with a protection protein derived from pIgR. This invention produces transgenic plants that assemble secretory immunoglobulins with great efficiency. The present invention makes passive immunotherapy economically feasible.